1. Field of the Invention
The present invention applies to the field of wireless communications systems and, in particular, to two-way radio communications using squelch codes.
2. Description of the Prior Art
Personal Radio Service (PRS) is the name applied by the Federal Communications Commission Wireless Telecommunications Bureau to cover a broad array of two-way (or, walkie-talkie) type radio services. Included under the PRS moniker are radio services such as the so-called 218-219 MHz service, the Citizens Band (CB) radio service, the Family Radio Service (FRS), the General Mobile Radio Service (GMRS), the Low Power Radio Service (LPRS), the Medical Implant Communications Service (NHCS), the Radio Control Radio Service, and the Wireless Medical Telemetry Service.
These “two-way” radios, sometimes referred to as walkie-talkies or handie-talkies, generally broadcast directly from one communications device (typically a portable radio) to another, or they may use a repeater to amplify their signals. These devices generally use allocated channelized spectrum. These channels can be frequency bands around a central carrier. For example, the spectrum allocated to FSR has been divided into fourteen (14) channels from 462.5625 MHz through 462.7125 MHz, and again from 467.5625 MHz through 467.7125 MHz, with each channel occupying a 30 KHz frequency band (see, e.g., 47 CFR §95.627).
Since anyone can use the allocated channels at any time, when a radio is tuned to a particular channel, a user can hear anyone who transmits on that channel. Also, if two people transmit simultaneously on a given channel, the radio frequency (RF) signals interfere with one another, and the listeners will likely hear noise. Thus, a user has to listen to-the conversations of other parties with whom the user does not wish to communicate if these parties are using the same channel. Furthermore, if these other parties communicate simultaneously with the user, or with people the user wishes to communicate with, this will interfere with the users ability to communicate.
To address the problem of listening to the conversations of strangers, some manufacturers of such two-way communication devices enable the use of squelch codes. Squelch is the phrase used for muting a speaker. For example, two-way radios generally squelch the speaker until a minimum signal strength is detected, so that the radio is not constantly humming with background noise. By using squelch codes, the radio only un-squelches the speaker when it detects the squelch code. Examples of squelch codes are Continuous Tone Coded Squelch System (CTCSS) sub-audible tones which are out-of-band tones and Digital Coded Squelch (DCS) code words, which are digital squelch codes. Motorola's trademarked version of these squelch systems are called Private Line (PL)™ and Digital Private Line (PL)™ respectively.
As an example, FSR uses CTCSS to create “sub-channels.” Each user selects which CTCSS tone to use in conjunction with a selected RF channel. The radio then blocks, i.e. squelches, any signal received that does not include the selected CTCSS tone. Thus, if two users select a CTCSS tone not used by other parties, then the two users will only hear transmissions from each other. However, the use of squelch codes does not solve the problem of interference. Two users transmitting on the same channel at the same time using different squelch codes will still interfere with one another, leaving the receiver unable to render the desired content.